Chemical cleaning of Alum scale from water treatment piping

[tomecki]

Our water treatment flocculant supply line is severely scaled up with what is believed to be Calcium Sulfate scale, thought to have formed while the plant used Alum (We have made the switch to PACl since.)

Lab test have been performed on the scale, and Methanesulfonic Acid and Green Liquor (pulping chemical) have been proposed for the cleaning. We can isolate the line, so that theoretically none of the chemical will get into the water assuming it's flushed properly.

I'm not very familiar with the regulations surrounding potable water safety so I'm looking for some advice.

Are we allowed to use these chemicals in the water system even though they're not NSF approved?
If so, how do we flush and prove that the pipe is clean enough to be returned to service?
Does anyone have experience with Alum scale and can suggest a better chemical for taking care of it?
Links to resources about this subject would be appreciated.

Thanks!

 

[bimr]

Are you sure that it is calcium sulfate and not alum crystals? A build-ups of alum crystals sometimes forms by evaporation processes. Under certain circumstances crystal formation can "seed out" additional dissolved aluminum sulfate and create serious plugging problems.

Calcium sulfate scale typically occurs at a higher pH above 10, than the pH of aluminum sulfate (pH 2.3)

A low pH cleaning solution (target pH of 2.5) of 0.5% (w) of HCL (hydrochloric) acid can be used. It is useful in removing inorganic scale (e.g. calcium carbonate, calcium sulfate, barium sulfate, strontium sulfate and metal oxides/hydroxides (e.g. iron, manganese, nickel, copper, zinc) and inorganic-based colloidal material. HCL acid, a strong mineral acid, is also known as muriatic acid. HCL acid is available in a number of concentrations.

Methanesulfonic Acid is an organic acid and would contaminate potable water. Would expect that Green Liquor (pulping chemical) would be higher pH and would not work at all.

While an acid would remove the calcium sulfate, there are other issues. Will the piping withstand the acid? What will you do with the waste stream? Who will be doing this work with corrosive and hazardous chemicals?

Unless there are unusual circumstances, it would seem like a better approach would be to replace the pipe with new pipe.

 

[tomecki]

I have found the answer to part of my original question regarding the use of cleaning chemicals: NSF-60 says that anything that comes in contact with piping or equipment that will later come in contact with drinking water has to be NSF approved. When it comes to cleaning chemicals, they're usually approved only for "off-line" use and have a flushing/cleaning procedure as part of the certification. Neither of the chemicals I mentioned are certified.

bimr: I'm not 100% sure what the scale is. Calcium sulfate has been proposed as being the most likely by some of our more chemically minded people. We don't think it's Alum crystals because the scale only formed downstream of the carrier water injection point. Not in the Alum tanks or in the part of the line upstream of the carrier water injection point. The thinking is that the scale formed as the Alum contacted the carrier water and formed precipitates, most likely from minerals dissolved in the water.

We've tried HCl and H2SO4 on scale samples in the lab and they didn't work.

Part of the line is buried under ground and part of it runs through the clarifier, which we can't take offline until winter, so replacement is not an option.

As I mentioned before, we're a pulp mill and are well equipped to handle and dispose of nasty chemicals so that's not a problem.

If anyone has seen this before, or has any other suggestions on which NSF-60 chemicals might work, I would appreciate the input.

 

[bimr]

NSF is a voluntary standard for the most part.

The minerals in the water that form scale are hardness, calcium carbonate, magnesium carbonate, calcium sulfate. These scale formers will readily dissolve in acid, so they are not likely to be the source of the scale. The water scale also form at high pH (above 10+) which you do not have. Calcium sulfate solubility is over 2,000 mg/L. so it not likely to be the scale.

Aluminum sulfate may is made by adding aluminum hydroxide, Al(OH)3, to sulfuric acid, H2SO4:
2 Al(OH)3 + 3 H2SO4 → Al2(SO4)3·6H2O

When dissolved in a large amount of neutral or slightly alkaline water, aluminum sulfate produces a gelatinous precipitate of aluminum hydroxide, Al(OH)3.

Aluminum hydroxide is amphoteric. It dissolves in acid, forming [Al(H2O)6]3+ (hexaaquaaluminium) or its hydrolysis products. It also dissolves in strong alkali, forming [Al(OH)4]− (tetrahydroxidoaluminate).

You should consider increasing the velocity and shortening the distance of your carrier pipes.

 

[tomecki]

Thanks for trying to help but I'm not sure what to make of your response. I'm a mechanical engineer and my knowledge of chemistry is insufficient to make sense of what you're trying to say.

As I mentioned before, neither Hydrochloric Acid nor Sulfuric Acid dissolved the scale in the lab. Do you think other acids may work? I'll see if they can try some strong alkali in the lab too.

Changing the length and diameter of the pipe is impractical. Also, we don't use Alum any more so I'm not concerned with solving the root cause of this problem.

 

[bimr]

NSF is a voluntary standard which you have no legal responsibility to follow. NSF is an independent testing and certification that manufacturers pay for NSF certifications. However, using a NSF certified product provides a guarantee that the product has been tested.

The scale is aluminum hydroxide, Al(OH)3 precipitate.

Aluminum hydroxide is amphoteric. It dissolves in acid, forming [Al(H2O)6]3+ (hexaaquaaluminium) or its hydrolysis products. It also dissolves in strong alkali, forming [Al(OH)4]− (tetrahydroxidoaluminate).

The scale should dissolve in acid or strong base.

http://www.public.asu.edu/~jpbirk/qual/qualanal/aluminum.html

Did you imply that green liquor (a strong alkali) dissolved the scale?

 

[mell9401]

I'll offer a little clarification regarding NSF...the legal responsibility to use NSF certified products will depend on your locality and application. In Washington state, for example, they require that certain materials and chemicals used in drinking water treatment be NSF approved, although they grant several exceptions:

http://app.leg.wa.gov/WAC/default.aspx?cite=246-290-220

A quick call to your local drinking water regulator should be able to clarify the requirement for your specific application.